1. Field of the Invention
The present invention relates generally to bearing assemblies for a drilling motor. In particular, the present invention relates to downhole oil-sealed bearing pack assemblies for a drilling motor.
2. Description of the Related Art
In the oil and gas industry, as well as in mining and other industries, holes are often drilled into the earth to reach the desired stratum to evacuate natural resources. To drill deep holes, the practice of using a fluid motor to drive a drill bit has become commonplace. In operation, the fluid motor is installed at the lower end of a drill pipe string and drilling fluid or mud is circulated down through the drill string and motor. The drilling mud flowing through the motor causes a mounted driveshaft to rotate. A drill bit, which contains nozzles, is secured to the end of the drive shaft and rotates to cut through the formation or stratum. Simultaneously, the drilling mud passes through the bit nozzles to flush away the cuttings. Once the drilling mud has exited the nozzles, the mud and cuttings return to the drilling rig or surface through the annulus created between the outside of the drill pipe string and the borehole.
During well drilling operations, the drill bit is forced against the earth's formation by the weight of the drill string. The weight of the drill string is transferred through a rotatable bearing assembly to a hollow drive shaft which is attached to the drill bit. In general, the drive shaft is driven or rotated by the rotor of the fluid motor. A bearing housing, containing the rotatable bearing assembly and through which the drive shaft extends, remains relatively stationary. As a result of this drilling method, the rotatable bearing assembly must endure severe vibration, shock, and axial and radial loading.
Typically, fluid motor bearing assemblies include a combination of bearing elements, such as radial bearings and thrust bearings. The rotation of the drive shaft within the bearing assembly creates a substantial amount of heat within the individual bearing elements. As a result, the bearing elements must be cooled by some type of lubricant.
In the past, one technique for cooling the bearing assemblies was by allowing a small portion of the drilling mud to circulate through the bearing elements. A portion of the drilling mud in the drill string was diverted from the hollow drive shaft to the bearing assembly. Although this method of cooling was effective, it had the disadvantage of introducing the polished bearing elements to abrasive particles, such as mud, grit and formation cuttings. The abrasive particles caused excessive wear on the bearings and reduced their effectiveness and life expectancy. Another disadvantage with mud cooled or lubed thrust bearings was the necessity of spherical rolling elements, as opposed to cylindrical rolling elements, due to grit and debris in the mud. The presence of grit in the mud causes cylindrical rolling elements to slide, rather than roll. A disadvantage with mud cooled thrust bearings with spherical rolling elements was that spherical rolling elements have a lower load capacity than cylindrical rolling elements.
By contrast, other prior art fluid motor bearing assemblies were cooled by an oil or grease type lubricant. The oil-sealed bearing assemblies were sealed at opposite ends of an annular bearing chamber existing between the drive shaft and the bearing housing. Seals were necessary to prevent drilling mud from entering into the oil-filled bearing chamber from the mud-filled drill string. Sealing this system, however, was difficult because the pressure of the drilling mud within the drill string and drill motor was often 2,000 pounds per square inch (psi) greater than the drilling mud pressure after exiting the nozzles of the drill bit. Thus, the disadvantage of this system was that for the seals to protect the oil-filled bearing chamber from drilling mud, the seals needed to be able to seal the 2,000 psi differential across the seal. As a result, the life expectancy of these seals was very low and failures occurred frequently.
Another method of sealing drilling mud from the oil-filled bearing chamber was to employ a low pressure seal and create a hydraulic pressure drop within the drill motor such that the low pressure seal only needed to seal a pressure differential of a few pounds per square inch. A mechanical face seal or flow restrictor was used to reduce the pressure near the bearing chamber seals to approximately the pressure found within the borehole annulus between the borehole and the drill string. The mechanical face seal permitted drilling mud to flow from the drill string out to the borehole annulus. The mechanical face seal included two mating surfaces that were in sliding contact during drilling operations. One of the mating surfaces was secured to the stationary bearing housing and the second mating surface was attached to the rotating drive shaft. Drilling mud would leak between the two contacting surfaces causing a gradual pressure drop from the high pressure of the drill string to the low pressure of the borehole annulus. The disadvantage of this system included wear of the mating surfaces due to their sliding contact. Another disadvantage was that the fluid which leaked across the mechanical face seal needed to be nonabrasive to minimize the erosion of the mating surfaces.
Oil-sealed bearing assemblies, like those described above, typically used seals that contacted the surface of the rotating drive shaft. Usually, the seals were made from an elastomeric material. Because the seals were in contact with the rotating drive shaft, the drive shaft was coated with a special coating to reduce wear on the contact surface.
Coating the drive shaft has several disadvantages. For example, since the drive shaft is often under severe bending and torsional loading conditions during operation, applying any type of coating to the drive shaft reduces the shaft's fatigue life and increases the probability of fatigue failure. Another disadvantage of coating the drive shaft manifests itself when the coating becomes worn and the drive shaft must be taken out of service to be recoated. During the period of time in which recoating occurs, another expensive drive shaft is required to put the apparatus back into operation. Thus, an operator would need an inventory of expensive replacement drive shafts to drill with a coated drive shaft.
Alternatively, some oil-sealed bearing assemblies attached a wear sleeve to the drive shaft. The wear sleeve was fit onto the drive shaft and the seals contacted the wear sleeve rather than the actual drive shaft. The disadvantage of this system was the excessive heat generated at the seal and wear sleeve interface which caused the seals to overheat and fail. This excessive heat did not usually occur in the drive shaft/seal combination because the circulating mud within the bore of the drive shaft dissipated the heat at this interface.
Typically, an oil-sealed bearing assembly included an oil reservoir and a floating piston on top of the reservoir to pressure compensate between the lubricating oil and the drilling mud. Additionally, the floating piston included a seal and a roller bearing which contacted the rotating drive shaft. Because the piston floated on top of the oil reservoir, it permitted the oil to thermally expand within the reservoir while simultaneously providing pressure to the oil within the reservoir to compensate for any oil loss across the seals.
A disadvantage of the floating piston was its tendency to bind between the drive shaft and the bearing housing as the drive shaft bent in response to side loadings. Another disadvantage included the roller bearing scarring the surface of the rotating drive shaft in the area which the seals contacted the drive shaft. Yet another disadvantage of this system included the absence of a means for checking the oil level within the reservoir while out on a rig or platform.
An oil-sealed bearing pack assembly is desired to overcome the disadvantages of the pack assembly described above. Such a bearing pack assembly should reduce the differential pressure across upper and lower seals of the bearing pack. Further, it should reduce the wear on the shaft. Additionally, the bearing pack assembly should provide a means for easily checking the oil reservoir level.